SOME NOTES ON THE METALLURGICAL 

 PHOTOMICROSCOPE. 



By J. H. G. MONYPENNY. 



(Chief of the Research Laboratory, Brown Bayley's Steel 



Works, Ltd.) 



The technique of the photomicrography of metals has advanced 

 very much during the last ten or twenty years, but there are still 

 very marked evidences that many who take up microscopic work 

 in connection with metallurgy appear to study the microscope itself 

 either not at all or only to a very small extent. The consequence 

 is that statements are made about the structures of various metals 

 which are not correct; the presence in sections of minute particles 

 or membranes of constituents other than those stated to be there 

 has been missed simply because the operator did not know how to 

 use his microscope properly. Again, photographs are published 

 which have only a slight resemblance to the structures photographed, 

 in some cases the definition is so bad that the reproductions are 

 not worth the paper they are printed upon. One has only to 

 look through the Journals of, for example, the Iron and Steel 

 Institute to see how true this is. 



Even when a metallurgist has devoted a considerable time to the 

 study of the microscope, mistakes may arise in the interpretation of 

 structures. For example, it has been stated that iron carbide 

 (cementite) is not attacked by sodium picrate when its thickness is 

 less than 0.001 mm. (this statement is repeated in one of the most 

 recently published treatises on metallography). This is quite incor- 

 rect. Not only are the carbide laminae of pearlite attacked when 

 considerably thinner than this (certainly not more than one-tenth of 

 the thickness mentioned), but also the minute granules in sorbite, 

 produced on tempering hardened steel at about 600° C. Possibly 

 the reason the above misstatement was originally made was either 

 that the aperture of the objective used was not sufficiently high or 

 that the resolving power was much reduced by the use of a prism 

 illuminator or both. 



In the following pages the author has attempted to set out 

 some of the conditions which appear to him to be necessary to secure 

 good photomicrographs of metals and the means he has devised from 

 time to time to fulfil these conditions. 



(a) The lllmniiKint and Condensing Si/stem. — Few who have had 

 any experience in photomicrography will disagree with the statement 

 that the illumination of the specimen is of fundamental importance 

 in the production of a good photomicrograph. Good illumination 

 should comply with the four following conditions: — 



(1) The whole surface which is required to be reproduced should 



be evenly illuminated. 



(2) The lighting should be such that the whole aperture of the 



objective may be utilised. 



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